In a perfect world, in-laws would welcome new members of their families and strive to get along.  In reality, things can be great or go the extreme found largely in India where brides are sometimes burned alive if they don't meet financial or other expectations.  

If your relations with your relatives by marriage are not as good as they should be, here are some tips that may help you all just get along.

Put the welfare and safety of a child before any other consideration.

Do not try to jockey for control of your loved one.   Family is not a contest.

Distance yourself emotionally.   You do not have to automatically love those your loved one loves.   If you grow to care for each other, that is a bonus and a blessing.

Let go of your children even if you dislike their choice of partner.   They will generally come back to you with love and respect.  And you may be wrong about the partner.  Or not.

Don't ask your loved one to choose.   If the relative has emotional problems, ask your loved one to step back and look around for solutions regarding the relative's behavior.   In other words, do not come between your loved one and his/her relative if you can help it.   This isn't always easy, but it will decrease conflict.

Ask for your loved one's support in a logical and reasonable way.   A loved one who will not stand up for you might be willing to keep you and the in-law separated by not attending a family function or moving to a location that is farther away.

If you have done or said something wrong, apologize from the heart and correct your behavior.

Discuss, don't accuse.   Don't add fuel to the fire by making negative comments.   Always be polite and as pleasant as you can.   When you have been pushed too far, say so calmly.   Walk away if you have to.

Playing deaf might prevent useless arguments.

When dealing with irrational people, don't respond irrationally.  If you can't convince your loved one to support you, that's where the real problem lies.

Marriage counseling might help.   And if you can get your in-law to go, family counseling is an option.